The present invention relates generally to oxide high-temperature conducting wires and methods of producing the same and in particular to oxide high-temperature superconducting wires for use in the fields of electric power, transportation, high energy, medical field and the like and methods of producing the same.
In recent years it has been reported that an oxide sintered compact exhibits superconducting properties at high critical temperature, and such superconducting properties are being employed to promote practical use of superconduction technology. It has been reported that a yttrium-based oxide presents superconduction at a temperature of 90k and a bismuth-based oxide at a temperature of 110k. These oxide superconductors are expected to serve practical use as they exhibit superconducting properties in liquid nitrogen, which is available at relatively low cost.
To allow such a superconductor to pass ac current to supply power, the superconductor is sheathed with silver, the silver sheath is coated with a high-resistance element and the high-resistance element is also coated with metal to provide a superconducting wire to pass ac current.
The superconductor is sheathed with silver in order to prevent the superconductor from having a crystal structure impaired when it is for example drawn. The silver sheath is coated with the high-resistance element in order to reduce ac loss. Furthermore, the high-resistance element is coated with metal in order to obtain a level of elasticity required for the superconducting wire.
To allow such a superconducting wire as described above to serve practical use, its critical current value needs to be improved. In particular, to use the superconducting wire for a cable, ac equipment for example for a transformer and the like, not only is its critical current value required to be improved but also its ac loss must be reduced. To achieve this, the silver sheath is covered with a high-resistance element formed of a material carefully selected.
Such a superconducting wire structured as above is disclosed for example in International Publication WO96-28853 and Japanese Patent Laying-Open No. 10-50152. 
International Publication WO96-28853 discloses a method of producing an oxide high-temperature superconducting wire, wherein an oxide superconductor is circumferentially coated for example with silver and thereon further circumferentially coated with a metal which is in turn oxidized to form a high-resistance element formed of a metal oxide between the silver and the metal.
Japanese Patent Laying-Open No. 10-50152 discloses a method of producing an oxide high-temperature superconducting wire, wherein an oxide superconductor is coated with silver and then thereon circumferentially coated with a resistive alloy (a high-resistance element) which is in turn oxidized to provide an insulating oxide between the silver and the resistive alloy.
In the techniques disclosed in the above two documents, however, a metal or a resistive alloy is oxidized to provide a high-resistance element or an insulating oxide. In this oxidization process, the metal or the resistive alloy has a component diffusing into the superconductor and thus changing its crystal structure. This disadvantageously results in the superconducting wire having a reduced critical current density. As such, while a high-resistance element is arranged to reduce ac loss, the method of producing the same and the material(s) for and type of the same can disadvantageously affect and thus reduce the critical current density of the superconducting wire, an essential property thereof. Thus, there is a need for a development of a material for the high-resistance element and of a method of producing the same which does not have an effect impairing superconducting properties of the superconducting wire including critical current density.
One object of the present invention therefore is to overcome such disadvantages as described above, and provide an oxide high-temperature superconducting wire having a structure with an oxide superconductor surrounded by a high-resistance element prevented from having an effect impairing on superconducting properties including critical current density, and a method of producing the same.
The present invention in one aspect provides an oxide high-temperature superconducting wire including an oxide superconductor, a sheath, a high-resistance element and a coating. The sheath is formed of a material containing silver and covers the oxide superconductor. The high-resistance element is formed of a strontium-vanadium oxide and coats the sheath. The coating is formed of a material containing silver and coats the high-resistance element.
In the oxide high-temperature superconducting wire in the above one aspect the high-resistance element, formed of a strontium-vanadium oxide and arranged to reduce ac loss, does not have a negative effect on the oxide superconductor or the sheath to impair superconducting properties. Thus, ac loss can be reduced and, as a result, critical current density can be improved.
The present invention in another aspect provides an oxide high-temperature superconducting wire including an oxide superconductor, a high-resistance element and a coating. The high-resistance element is formed of ceramic and coats the oxide superconductor. The coating is formed of a material containing silver and coats the high-resistance element. The ceramic is preferably formed of a strontium-vanadium oxide.
The present invention in still another aspect provides an oxide high-temperature superconducting wire including an oxide superconductor, a first high-resistance element, a sheath, a second high-resistance element and a coating. The first high-resistance element is formed of ceramic and coats the oxide superconductor. The sheath is formed of a material containing silver and covers the first high-resistance element. The second high-resistance element is formed of ceramic and coats the sheath. The coating is formed of a material containing silver and coats the second high-resistance element. The ceramic is preferably formed of a strontium-vanadium oxide.
In the oxide high-temperature superconducting wires in the above two aspects the high-resistance element and the first high-resistance element, formed of ceramic and arranged to reduce ac loss, do not have an effect for example impairing superconducting properties if they are brought into contact with a surface of the oxide superconductor. Thus the high-resistance element and the first and second high-resistance elements can provide their essential function of reducing ac loss and as a result improve critical current density.
The present oxide high-temperature superconducting wires in the above three aspects preferably include a plurality of sheaths, with the high-resistance element or the second high-resistance element interposed therebetween. Thus, ac loss can further be reduced.
Preferably the oxide superconductor is provided in the form of a filament.
Preferably the high-resistance element and the first and second high-resistance elements are formed of the strontium-vanadium oxide including at least one of Sr6V2O11 and SrV2O6.
Preferably the sheath and the coating are formed of silver or silver alloy.
Preferably the oxide superconductor is a Bi(Pb)xe2x80x94Srxe2x80x94Caxe2x80x94Cuxe2x80x94O-based superconductor.
The present invention in one aspect provides a method of producing an oxide high-temperature superconducting wire including the three following methods:
Method I of Producing an Oxide High-temperature Superconducting Wire
This method includes the steps of:
(a) packing, in a first pipe formed of a material containing silver, source powder providing an oxide superconductor when the source powder is thermally treated or powder of the oxide superconductor;
(b) arranging in a second pipe formed of a material containing silver the first pipe having the source powder or the powder of the oxide superconductor packed therein;
(c) packing a powdery strontium-vanadium oxide between an outer circumferential surface of the first pipe and an inner circumferential surface of the second pipe; and
(d) subjecting to a plastic working and a thermal treatment the second pipe having the powdery strontium-vanadium oxide packed therein.
Method II of Producing an Oxide High-temperature Superconducting Wire
This method includes the steps of:
(a) packing, in a first pipe formed of a material containing silver, source powder providing an oxide superconductor when the source powder is thermally treated or powder of the oxide superconductor;
(b) preparing from a powdery strontium-vanadium oxide a green compact having a hole;
(c) inserting into the hole of the green compact the first pipe having the source powder or the powder of the oxide superconductor packed therein;
(d) arranging in a second pipe formed of a material containing silver the green compact having the first pipe inserted therein; and
(e) subjecting to a plastic working and a thermal treatment the second pipe having the green compact packed therein.
Method III of Producing an Oxide High-temperature Superconducting Wire
This method includes the steps of:
(a) packing, in a first pipe formed of a material containing silver, source powder providing an oxide superconductor when the source powder is thermally treated or powder of the oxide superconductor;
(b) applying on an outer circumferential surface of the first pipe having the source powder or the powder of the oxide superconductor packed therein a slurry prepared from a powdery strontium-vanadium oxide;
(c) arranging in a second pipe formed of a material containing silver the first pipe having the slurry applied thereon; and
(d) subjecting to a plastic working and a thermal treatment the second pipe having the first pipe arranged therein.
Furthermore the present invention in another aspect provides a method of producing an oxide high-temperature superconducting wire including the following method:
Method IV of Producing an Oxide High-temperature Superconducting Wire
This method includes the steps of:
(a) preparing a green compact in the form of a bar from source powder providing an oxide superconductor when the source powder is thermally treated or from powder of the oxide superconductor;
(b) applying on a surface of the green compact a slurry prepared from a powdery strontium-vanadium oxide;
(c) inserting into a first pipe formed of a material containing silver the green compact having the slurry applied thereon;
(d) arranging in a second pipe formed of a material containing silver the first pipe having the green compact inserted therein; and
(e) subjecting to a plastic working and a thermal treatment the second pipe having the first pipe arranged therein.
Furthermore the present invention in still another aspect provides a method of producing an oxide high-temperature superconducting wire including the following two methods:
Method V of Producing an Oxide High-temperature Superconducting Wire
This method includes the steps of:
(a) preparing a green compact in the form of a bar from source powder providing an oxide superconductor when the source powder is thermally treated or from powder of the oxide superconductor;
(b) applying on a surface of the green compact a slurry prepared from a powdery strontium-vanadium oxide;
(c) inserting into a first pipe formed of a material containing silver the green compact having the slurry applied thereon;
(d) applying on outer circumferential surface of the first pipe having the green compact inserted therein a slurry prepared from a powdery strontium-vanadium oxide;
(e) arranging in a second pipe formed of a material containing silver the first pipe having the slurry applied thereon; and
(f) subjecting to a plastic working and a thermal treatment the second pipe having the first pipe arranged therein.
Method VI of Producing an Oxide High-temperature Superconducting Wire
This method includes the steps of:
(a) applying on an inner circumferential surface of a first pipe formed of a material containing silver a slurry prepared from a powdery strontium-vanadium oxide;
(b) packing, in the first pipe having the slurry applied thereon, source powder providing an oxide superconductor when the source powder is thermally treated or powder of the oxide superconductor;
(c) applying on an outer circumferential surface of the first pipe having the source powder or the powder of the oxide superconductor packed therein a slurry prepared from a powdery strontium-vanadium oxide;
(d) arranging in a second pipe formed of a material containing silver the first pipe having the slurry applied thereon; and
(e) subjecting to a plastic working and a thermal treatment the second pipe having the first pipe arranged therein.
In methods I-VI preferably the powdery strontium-vanadium oxide has a grain size of 1xcexcm to 10 xcexcm.
In methods I-VI preferably the step of subjecting includes twisting the second pipe before compressing and thermally treating the second pipe.
Thus in the present invention a high-resistance element arranged to reduce ac loss can be formed of a strontium-vanadium oxide to effectively reduce ac loss without impairing superconducting properties including critical current density.